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What is jitter and what does it mean?

Discussion in 'Currawong' started by currawong, May 15, 2009.
  1. Currawong Contributor
    I've got to admit, I didn't properly understand what digital jitter is and how it affects sound. There's been quite a bit of discussion about it, so here is a collection of articles that explain it:

    Steve Nugent of Empirical Audio explains in easily understandable terms what jitter is and where it matters. The easiest article to read. You'll need to know the basics of digital transmission though, such as what S/PDIF, AES/EBU and optical digital are, for example.

    HDD Audio asks, Can you Hear Jitter? They also created files for a second test. However, I wonder, if you have a lo-fi system with lots of jitter, would you really hear a difference?

    Maybe more usefully, for those of you who think digital is just 0's and 1's, have a look at the picture below, from high-end digital gear, showing how digital waveforms, after they pass through equipment, aren't exactly "on" or "off any longer, but have measurable jitter:


    Edit: There's also some discussion from Dan Lavry here which is interesting, as it discusses the actual harmonic effects of jitter. The subsequent posts in that thread from him are also interesting.
    Champ HkGt likes this.
  2. Omega17TheTrue
    Interessing, tanks you.
  3. gregorio
    I think you may have missed the main point. Although your graph gives a visual representation of jitter, what you haven't considered is that DACs have jitter removal circuitry (a PLL for instance) which will (for all intents and puposes) completely eliminate the jitter introduced by digital transmission. So transmission jitter is irrelevant and both blue and red traces on your graph would sound identical when converted to analogue.
    Champ HkGt likes this.
  4. Currawong Contributor
    gregorio: While in theory this may be correct, in practice it isn't so. Cheap DACs don't re-clock the signal from what I understand. Expensive ones may, however though, so it's dependant on the equipment.
    1mctous likes this.
  5. gregorio
    A PLL (for example) is not re-clocking the signal, it's effectively filtering the original signal. There are a number of methods used for jitter rejection in modern DACs, Re-clocking and PLLs are just two. I'm sure there are some examples of cheap DACs which have no jitter rejection circuitry of any kind but as time goes on there are fewer of these types of DACs. Pretty much any relatively new, decent DAC will have jitter rejection circuitry.
  6. Currawong Contributor
    The purpose of my post though wasn't to give so much as a definitive answer, but link to information so that people at various levels of understanding can investigate for themselves. [​IMG]
  7. RinksCustoms
    agreed with most above i am. id like to add alittle info to hopefully take the mystery out of waveform ringing. A digital signal entering a chip (like a DAC) will only cause audible "jitter" if the following conditions are met:
    1. the DAC's power supply isnt "weak" under full output (ie. well regulated to within 100mV or less - usually within 25-50mV of the target Vout).
    2. the input ringing does not cause a logic change, (ie. excessive ring amplitude to cause an input to swing low/high enough to be considered a logic low or a logic high when it is not desired).
    3. the specified input frequency/voltage is not exceeded on either the DAC or the traces leading to/from the DAC, which would cause excess ringing and possible old data to still exist on the traces & IC pins. Traces on any circuit board will have some amount of inductance & capacitance as does the thousands of transistors within the DAC IC itself.
    So audible "jitter" is most likely some kind of "hickup" in the bits that make a frame of audio in your audio file.
  8. 1mctous
    Those of us who grew up with over-the-air analog TV will remember how the picture would roll with some weak signals.  That happened because the vertical sync circuits couldn't separate the all-important pulses.  The better and more steady the sync pulses got, the more stable the picture.

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